The present invention relates generally to a method of measuring the position of a liquid surface within a vessel, and more particularly to a method of measuring liquid level using an optical fiber sensor.
There are a variety of methods of measuring liquid level, the most commonly-used non-fiber-optic methods being based on such physical phenomena as buoyancy, capacitance, ultrasonic waves, and pressure transmission.
Liquid level sensors based on buoyancy employ a buoyant float which moves with changing liquid level. A mechanical linkage or magnetic couple translates the float's up-and-down motion into a motion of a contact which is either open or closed, indicating whether the liquid level is above or below a specific level. Accuracy is typically limited to approximately a quarter of an inch.
In the capacitance approach to measuring liquid level, an electrode is installed in the vessel and the capacitance between this electrode and the wall of the vessel is measured. Air has a dielectric constant of one and the liquid has a greater dielectric constant. As the tank fills with the liquid, the dielectric constant rises and therefore so does the capacitance. Therefore, a measure of the capacitance of the vessel is an indication of the amount of liquid present. Any change in the dielectric constant of the liquid will cause an incorrect reading.
Ultrasonic techniques sense liquid level by measuring the time it takes for a pulsed high-frequency sound wave to travel from a transducer downward through the air at the top of the vessel, reflect off the surface of the liquid and return to the sensor. Accuracy is typically limited to about 0.25% of the full-scale reading. Ultrasonic sensors are not reliable in the presence of surface foam, and their functioning may be impaired by falling liquids, steam, and dense vapors and dust in the vessel.
Liquid level sensors based on pressure operate on the principle that the pressure at the sensor increases directly with the depth of the water above it. One such sensor is called a bubbler. In a bubbler, compressed air is forced down a tube which runs to the bottom of the vessel at a pressure which will cause it to bubble out of the end of the tube. That pressure is an indication of the depth of the liquid above the end of the tube. One disadvantage of this sensor is that the end of the tube can become clogged by the liquid.
Optical fibers have been used in liquid level sensing. For example, point sensors work on the principle of total internal reflection. Light is sent down an optical fiber and the amount of light that gets reflected back from the end of that fiber depends on whether or not the fiber end is in the liquid or above it. They are also susceptible to contamination of the end of the fiber, and would not work in any kind of liquid that could stick to the end of the fiber. A variation of this type of sensor has a U-shaped fiber with the cladding stripped away from the U-shaped portion. When the U-shaped portion of the fiber is immersed in the liquid, and light is transmitted through the fiber, some of it is lost to the liquid. Therefore, the amount of light that is received depends on whether the U-shaped portion of the fiber is in or above the liquid. Both of these sensors have the limitation that they merely tell you whether the liquid level is above or below a specific level.
A differential absorption optical fiber liquid level sensor uses a two-wavelength ratiometric approach to cancel out errors arising from variations in fuel characteristics and tank vibrations. It uses inexpensive LED sources and a multimode optical fiber and can have a 2-mm resolution over an 18-cm range. The sensor also has the advantage that only a transparent window is needed to look up through the liquid to measure the transmission through the liquid, making the method non-contact and therefore not subject to surface contamination or surface wetting of the optical surfaces. A disadvantage of this sensor is that if the absorption characteristics of the liquid are temperature-dependent, then the reading must be adjusted for that property. Of course, it will not work on liquids which do not transmit light.
Another optical fiber sensor is based on the continuation of the transmission through a bent fiber. Fibers formed with reversed curvatures of decreasing radii will induce an increasing amount of lower-mode light loss to the cladding as the light propagates along the multimode fiber. The sensor is arranged in the fluid in a vertical orientation so that the light travels along the fiber from the bottom or low point of the fluid to the top or the full point. As the fluid covers increasing lengths of the exposed fiber, it strips ever more power from the cladding. Data taken with this sensor show a monotonic decrease of output intensity as a function of increasing fluid level. This sensor has an accuracy of a few centimeters.
Crosstalk between two multimode optical fibers has also been used to sense liquid level. The cladding is removed from a portion of each of the fibers to expose their cores, and then they are aligned so that the exposed cores are adjacent to each other. Light is propagated through one the fibers. When liquid is present in the region between the cores light will couple from the one fiber into the other. This sensor is very accurate over a limited range. The disadvantages are that it is very susceptible to contamination and it will only work with a limited range of liquids which have the right index of refraction. Also, the amount of coupling changes as the index of refraction changes, which happens with temperature, requiring that the sensor be temperature-compensated.
A digital optical fiber liquid level sensor operates on the selective coupling at the liquid surface between a source waveguide and an array of digitally masked receiving waveguides. The receiving waveguides carry optical high-low signals to a remote detector in a discriminator circuit. This sensor is capable of liquid level resolution to several millimeters and it can operate over a total range of liquid levels of several meters. This sensor also depends on the index of refraction of the medium.
A high-precision remote liquid level measurement can be made using a combination of optical radar and optical fibers. This technique is similar to the ultrasonic technique in which liquid level is measured by measuring the length of time it takes for radiation to travel the distance from the source to the surface and back to a detector. This distance measurement is made using the technique of optical radar in which the phase of an amplitude-modulated lightwave reflected from a remote target is compared with that of the original phase of the launched beam. This technique enables measurements to be made ranging from 0.1 m to 5 m with an accuracy of about 1 mm throughout the range. This system has all the advantages of the non-contact techniques that were described previously. Its readings are adversely affected by foam and the presence of particles or droplets between the source and the surface that would cause reflections.
It can be seen that a need exists for a highly accurate, depth-continuous method of measuring liquid level which can be used in a corrosive environment, has no moving parts within the liquid-containing vessel, can be used with a variety of liquids, is insensitive to such liquid properties as index of refraction, dielectric constant, absorption characteristics, and light transmissivity, operates in the presence of foam and various contaminants, and functions even when the liquid surface is not horizontal